Generally, surface protective films are used for purposes to prevent wounds or stains mimed when they are adhered to bodies to be protected, that is, adherends, which are processed or returned, using pressure-sensitive adhesive layers applied to their sides.
For example, in polarizing plates used in panels of liquid crystal displays, adhesion sides with pressure-sensitive adhesive layers of surface protective films have been protecting by the films, which are released and removed later on inspection process. However, fine bubbles may be incorporated when the protective films are adhered. A number of bubbles are generated under high temperature or high pressure condition such as autoclave process to muse defects. In addition, when the polarizing plates are adhered to panels of liquid crystal displays, or when panels are transferred, fine bubbles are generated on surfaces of surface protective films and polarizing plates by transfer equipments. Thus, wetting properties of pressure-sensitive adhesives for surface protective films are of interest.
Recently, surface treatments of polarizing plates are performed by various means. Surface treatments of polarizing plates include antiglare layers, low refractive index layers and/or high refractive index layers, or stain resistant layers, each of which has different surface roughness and surface energy. Especially, the antiglare layer has higher surface roughness than that of a triacetylcellulose layer and is generally hydrophobic to have low surface energy. Therefore, when the surface protective films are adhered, the amount of resulting fine bubbles varies depending on their surface properties (hydrophilic or hydrophobic) and affects greatly on wetting properties of pressure-sensitive adhesive for protective films.
Meanwhile, to evaluate reliability of panels in liquid crystal displays to which polarizing plates are adhered, they are left under high temperature and high humidity conditions for a long time. At this time, mura may be generated on surfaces of protective films by additives added for improving wetting property of protective films, or stains of polarizing plates may be caused, with transferring additives to surfaces of polarizing plates, to raise panel defects.
Generally, mura means stains generated to surfaces by exterior condition and allowed to be visually detected. Said mura includes cases of generating water spots due to different hydrophilicity of ingredients on storing under moisture-heat resistant condition, causing opacity due to different compatibility of various mixtures, or causing different turbidity (haze) in colors or surfaces due to heterogeneous dispersion of additives. Therefore, when mura is generated, appearance is poor. Particularly, mura in protective films may contaminate surfaces of adherends to be protected so that mum is an important problem.
Meanwhile, when polarizing plates are stored in a state of adhering surface protective films thereto under high temperature or high temperature and humidity condition, curls of polarizing plates may be generated depending on properties of additives in the protective films. The polarizing plates are prepared by laminating triacetylcellulose layer above and below polyvinylalcohol films, and curls are generated by difference between interlayer shrinkage and swell. Then, the triacetyl cellulose layer protecting surfaces by adhering the protective film thereto, is influenced by shrinkage or swell, depending on whether the additive is hydrophilic or hydrophobic, to cause curls in polarizing plates, which has severer effects under high temperature or high temperature and humidity condition.
To improve wetting properties and stain resistance of said adherend, various types of plasticizers or surfactants have been used.
In Japanese Unexamined Patent Publication Nos. H06-128539 and 2005-314476, antistatic agents of polyether polyol compounds and alkali metal salts were added to inhibit bleeding the antistatic additives to surfaces of adherends. However, bleeding of antistatic agents could not be effectively inhibited in the above disclosures. Especially, there was a problem that mum might be caused under moisture-heat resistant condition due to hydrophilicity, a property that polyether polyol has.
In addition, it is described in Japanese Unexamined Publication No. 2005-338150that contamination of surfaces of an optical film can be prevented by containing an ionic antistatic agent in a pressure-sensitive adhesive layer of the protective film, and saponifying the surface of optical film or pre-treating it with corona or UV, and the like. However, in the above disclosure, further process of pre-treatment was not only required for preparing optical films, but the protective film had also problems in terms of adhesion reliability and release force due to treatment by aqueous alkali hydroxide or corona used in saponification.
On the other hand, Japanese Unexamined Publication No. 2005-326531 discloses a polarizing plate with a protective film inhibiting generation of curls by restricting water content in the polarizing plate with pressure-sensitive adhesive. The above disclosure indicated that cause of curls in the polarizing plate was the water content in the polarizing plate. However, it did not indicate that additives in the protective film adhered to the polarizing plate, affected on the water content.